ABSTRACT
Background: Molecular markers of insecticide resistance can provide sensitive indicators of resistance development in malaria vector populations. Monitoring of insecticide resistance in vector populations is an important component of current malaria control programmes. Knockdown resistance (kdr) confers resistance to the pyrethroid class of insecticides with cross-resistance to DDT through single nucleotide polymorphisms (SNPs) in the voltage-gated sodium channel gene. Methods: To enable detection of kdr mutations at low frequency a method was developed that uses polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA)-based technology; allowing rapid; reliable and cost-effective testing of large numbers of individual mosquitoes. This was used to assay mosquitoes from sites in lower Moshi; Tanzania. Results: Sequence-specific oligonucleotide probes (SSOP) were used for simultaneous detection of both East and West African kdr mutations with high specificity and sensitivity. Application of the SSOP-ELISA method to 1;620 field-collected Anopheles arabiensis from Tanzania identified the West African leucine-phenylalanine kdr mutation in two heterozygous individuals; indicating the potential for resistance development that requires close monitoring. Conclusion : The presence of the West African kdr mutation at low frequency in this East African population of An. arabiensis has implications for the spread of the kdr gene across the African continent
Subject(s)
Anopheles , Insecticide Resistance , MalariaABSTRACT
Human onchocerciasis is a debilitating disease caused by the filarial parasite Onchocerca volvulus (Leukart). The parasite is transmitted by dipterous insects of the family Simuliidae. ln West Africa, transmission is by members of lhe Simulium damnosum (Theobald) species complex. The manifestations of the disease are mainly dermal, lymphatic and ocular, the most severe of which is blindness hence the common name 'river blindness'. The disease is found in parts of Latin America, the Yemen and Sub-Sahara Africa. lt affects some 17.7 million people world-wide with 500,000 severely visually disabled and some 27O,00O blind. ln West Af rica, which is the most affected area, there are over 5 million infected with onchocerciasis and about 148,000 blind. ln view of this, the Onchocerciasis Control Programme in West Africa (OCP), one of the biggest vector control operations covering parts of 11 countries was started in 1975 by the World Health Organization to control onchocerciasis as a disease of public health and socio-economic importance. The initial strategy of the OCP was the application of insecticides to the breeding sites of the insect vectors. ln the OCP, temephos an organophosphate insecticide (OP) was the only chemical used for the initial five years due to its selective mode of action. Resistance to temephos first appeared in 1980 in a population of S. soubrense/S.sanctipauli on the Bandama river in Southern Côte d'lvoire. Later, similar resistant populations of the same species were identified in other river basins in the C6te d'lvoire. ln 1981 Chlorphoxim, also an OP, was used as a replacement for temephos in areas where resistance to temephos had been detected but a similar pattern of resistance was soon detected against chlorphoxim Careful monitoring of resistance to temephos has shown that it has spread to all river basins and different species of the S. damnosum complex at one time or another within the OCP area. Resistance to insecticides demands that large quantities and more novel and toxic chemicals have to be used in the fight against insects and other arthropod pests. This has important economic, operational and environmental demands. ln the West African vector control programme mentioned above, a rotation of 6 insecticides is used to manage insecticide resistance. This strategy places a major constraint on the operations of the Programme (OCP) due to the logistic need of stocking insecticides in the field